The present invention relates generally to liquid fuel tanks for airplanes and, more particularly, to liquid fuel tanks for airplanes which constitute a structural part of the airplane wing and which have an outer surface which defines the aerodynamic outer surface of at least a portion of the airplane wing, and methods and apparatus for manufacturing the same.
Airplane fuel tanks are subject to several special design requirements. For example, efficient utilization should be made of the structural space available. In this connection, in view of fire safety considerations as well as considerations relating to the location of the centroid, the tanks are usually situated in the airplane wings where, however, only limited structural space is available. Accordingly, the walls of the tank should preferably coincide with the outer cladding of the wing so that the best possible utilization of the available structural space can be achieved.
Additionally, an airplane fuel tank must be both strong and light. Thus, the tank must be sufficiently strong so as to tolerate the fatigue loads resulting from the motion of the airplane and from the liquid fuel splashing. Accordingly, the tank is desirably constructed of relatively thick sheet metal which is not prone to buckling or fatigue.
On the other hand, since it is quite important to reduce the flight mass of the airplane to a minimum, there is a tendency to use the thinnest possible sheet metal in the construction of the fuel tank. However, such construction subjects the tank to the possibility of failing through fatigue.
Furthermore, the fuel tank should be easily accessible to allow for servicing. In this connection, the tank should be detachable from the associated structure or at least provided with adequate servicing doors or the like. Structural provisions should be made to minimize or prevent the possibility of fuel leakage resulting from accidents, flying debris and the like. Of course, it is important to reduce the risk of major ruptures in the fuel tank which might occur during a crash of the aircraft.
Several different varieties of airplane fuel tanks are currently in use. For example, fuel tanks constructed by welding light sheet metal material and which are fixed within the airplane wing by means of metallic straps have been used for many years and, in fact, are still in use in some propeller driven airplanes. The advantages of such fuel tanks are that the same are detachable to facilitate servicing and are inexpensive in construction. However, such fuel tanks do not efficiently utilize the space within the wing since it is not possible to locate the tank as close as would be desired to the outer cladding of the wing. It has been suggested that such tanks present a safety hazard in that they are liable to burst or at least leak in the event of a crash or other accident. Furthermore, such tanks are relatively heavy since the outer cladding of the airplane does not contribute to the strength of the tank.
Another conventional airplane fuel tank currently in use is the so-called "integral tank". Such integral tanks include compartments confined by the cladding sheet and by the wing ribs, the cladding and ribs having been riveted using sealing compounds to define a liquid-tight tank. Integral tanks are presently the most common tank structure being used in relatively thick-skinned commercial and military airplanes. Such tanks are advantageous in that efficient use of available space is provided in combination with a relatively light weight of the structure. However, such integral tanks have drawbacks such, for example, difficulty in servicing since such tanks are not detachable and the necessity of placing sealing compounds when repairs to the tank are required. Furthermore, integral tanks are relatively prone to developing leaks due to flying debris or the like when used in relatively thin skinned, small airplanes. Still further, to obtain reliable liquid tightness for the tank requires close attention during the construction of the tank, while otherwise the interior structure need not be smooth.
Another type of airplane fuel tank presently in use is the rubber bag tank. In the use of such rubber bag tanks, a smooth-walled space, which need not be liquid-tight, is formed through the interconnection of the outer cladding and the ribs in the wing. A thin rubber bag is introduced into this space through a servicing hatch and bosses provided on the bag are pushed into openings provided in the interior space within the wing. Such rubber bag tanks have been in common use since the 1950's both in military and in small airplanes. The advantages of such rubber bag tanks include relatively good space utilization and safety from damage against both flying debris and in the event of a crash. The tank is also quite strong since the inner space and surrounding structure operate at a single entity. However, among the drawbacks of rubber bag tanks are that the rubber bags must be periodically replaced after a certain number of years and that the bags must be constructed utilizing special molds. Furthermore, the provision of rubber bags of course results in extra weight being added to the aircraft.
Airplane fuel tanks are also known which include a combination of the features discussed above. For example, fuel tanks comprising a hybrid of integral and welded tanks are relatively common in small planes. Typically, a tank of this type has a welded structure and is detachably connected to the surrounding structure by threaded fasteners. A portion of an outer surface of such tanks has been utilized to form a part of the wing's aerodynamic outer surface. The advantages and drawbacks of such hybrid tanks are essentially the same as those discussed above in connection with the integral and welded tanks.
The state of the art in connection with fuel tanks of the type described above is disclosed in U.S. Pat. No 3,420,477 wherein the fuel tank has a multi-ply wall which coincides with the wing cladding. However, in the construction of the fuel tank disclosed in this patent, the wall of the wing which functions at the same time as the wall of the fuel tank does not comprise a sandwich structure in the same sense as that term is used in connection with the present invention as described in greater detail hereinbelow. More particularly, a plurality of reinforcements and bracing elements are required in the construction of the fuel tank disclosed in U.S. Pat. No. 3,420,477. Such reinforcing and bracing elements complicates the construction of the tank and renders the same quite difficult since, for example, it is necessary to provide riveted joints at locations to which access is difficult. Due to the presence of the numerous reinforcing and bracing elements within the interior of the tank, such tanks cannot be manufactured by the method of the present invention described in detail below. Furthermore, the presence of the bracing and reinforcing elements detract from the space available within the tank for accommodating fuel. Still further, a risk of fuel leakage is introduced through the use of riveted joints necessitated by the bracing and reinforcing elements.